During the testing of a fabricated memory device, it is often discovered that certain portions of a memory array are faulty, unreliable or defective in some way. A typical way to correct this problem is to flag the region as defective by blowing fuses or anti-fuses which set hardwired latches associated with the defective region and which map the defective region to a non-defective fully-operable redundant region. With this re-mapping, all attempts to access the defective region will be redirected to the redundant region known to be properly working.
Many memory systems implement this re-routing on a column-by-column or row-by-row basis. However, non-volatile memory like flash memory lends itself well to a block redundancy approach where redundant blocks of memory cells are substituted for defective main memory blocks. Fuses or anti-fuses are associated with the redundant memory blocks and are programmed with the addresses of the defective blocks. The fuse or anti-fuse programming sets corresponding latches which are used to redirect accesses from a defective memory block to a redundant memory block.
Fuses or anti-fuses and their associated latches and program circuitry take up a considerable area of a memory die and, once programmed to specific addresses cannot later be reprogrammed. As a result, there is a limited number of defective main memory blocks which can be identified for mapping to redundant memory blocks. In order to add more defective address possibilities, additional fuse and/anti-fuse sets and associated latches are required which increases die area.